Okay, let's break this down and troubleshoot systematically. Don't panic! Ground fault errors on both circuits simultaneously suggest a common problem, not necessarily two independent cable failures. Here's a plan

1. Common Elements to Investigate First (Most Likely Causes)
Neutral Connection (if applicable)

Some thermostats, even those controlling 240V circuits, require a neutral for their internal electronics.

Verify that the thermostat neutral is properly connected. A missing or loose neutral can cause very weird behavior and possibly false ground fault readings. Even if the heating works without it, the thermostat's fault detection might be compromised. This is the first and easiest thing to check.

Wiring Mistakes

Double, triple, and quadruple-check your wiring at BOTH thermostats. Specifically:

Line and Load

Make absolutely certain you didn't accidentally switch the line and load wires.

Tight Connections

Are ALL connections tight? Loose connections can create arcing and voltage fluctuations, potentially triggering ground faults. Use a screwdriver and physically tighten every screw terminal.

Wire Insulation

Inspect the wire insulation where it enters the thermostat terminals. Is any insulation damaged or stripped back too far, potentially creating a short?

Thermostat Ground Fault Sensitivity

Some thermostats are very sensitive. It's possible they are tripping due to minor leakage that wouldn't normally be a problem. While unlikely in this scenario since it's both circuits, it's worth keeping in mind.

2. Thermostat-Specific Checks

Thermostat Type and Settings



Verify Compatibility

Are the thermostats rated for electric floor heating and the correct voltage (240V)?

Programming

Are any settings incorrect (e.g., temperature limits, sensor selection)? Check the thermostat's manual for troubleshooting guidance on ground fault errors.

GFCI Threshold

Does the thermostat have an adjustable GFCI trip threshold? If so, it might be set too low. However, proceed with caution before adjusting.

Floor Sensor



Wiring

As you mentioned, incorrect wiring of the floor sensor could cause problems. Make sure the sensor wires are connected to the correct terminals on the thermostat. Follow the thermostat's instructions.

Sensor Placement

Is the sensor positioned correctly? It should be between the heating cables, not directly on top of one.

Sensor Condition

The sensor itself could be faulty. Most thermostats have a resistance value chart for the sensor at different temperatures. You can use a multimeter to check the sensor's resistance and compare it to the chart.

Overheating

While direct overheating from a sensor issue might not immediately trigger a ground fault, it could damage the cable insulation over time, leading to a ground fault. It's less likely to be the instant cause but worth considering.

3. Cable and Circuit Checks (If the above doesn't reveal the problem)

Resistance Checks

This is where you'll need a multimeter and a bit of patience.

Cable Resistance

Disconnect the heating cable wires from the thermostat. Measure the resistance of each cable circuit. Use Ohm's Law (Voltage^2 / Watts = Resistance). In this case 240^2 / 3500 should be approximately 16.5 ohms. Check if it's close to the expected resistance. A significantly higher or lower resistance indicates a problem with the cable.

Ground Fault Test

THIS IS CRITICAL. With the cable completely disconnected from the thermostat, use your multimeter to measure the resistance between each cable wire and the ground wire. This tests for a short to ground. You should see infinite resistance (or very high resistance) on both legs. If you see a low resistance value (anything less than several megaohms), that means the cable insulation is compromised and you have a ground fault.

Wiring from Panel to Thermostat

Verify that there are no nicks in the wire, crushed conduits, or other damage to the wiring between the panel and the thermostats.

4. Possible Scenarios and Causes

Cable Damage During Installation

Although unlikely that both cables would fail similarly at the same time, it's possible the cables were nicked or damaged during the initial installation of the heating system.

Moisture

Has there been any recent water exposure to the floor? Moisture can lower the insulation resistance and trigger a ground fault.

Voltage Spike

A voltage surge or spike on the electrical system could have damaged the cables, although this is less likely.

Existing Ground Fault

It's possible there was a very minor ground fault existing before you installed the thermostats, and the new thermostats are simply more sensitive and detecting it.

Steps to Take When You Go Back


Gather Information

Thermostat model number(s) and manufacturer. Heating cable manufacturer and model. Talk to the homeowner. Ask about any recent work done in the area (plumbing, renovations, etc.).2.

Inspect the Thermostats

Remove the thermostat covers and carefully examine the wiring.3.

Perform the Wiring Checks

As described above (neutral, line/load, tightness).4.

Perform the Resistance Checks

Most important to diagnose cable failure!5.

Document Everything

Take detailed notes and photos of your findings.

Important Safety Notes



Turn off the power at the breaker before working on any electrical wiring.

If you are not comfortable performing electrical troubleshooting, consult with a qualified electrician.

When testing for ground faults, disconnect the cable completely from the thermostat.*I know it sounds like a lot, but systematically checking each of these areas will help you isolate the problem. Start with the simple things first (wiring, thermostat settings) and then move on to the more complex checks (resistance tests). Good luck, and let me know what you find! Flag for review